Normally, a fluidized bed reacting apparatus comprises a chamber having inlet means for the powdered product to be processed and at least one perforated powder supporting plate arranged in a substantially horizontal plane in said chamber, as well as means arranged in a part of the chamber below said plate for producing a flow of gas upwardly through the perforations in said plate for the formation of a fluidized layer of said product through part of the height of said chamber overlying the perforated plate.
Fluidized bed reactors of this kind are widely used in industry for drying processes, which are often carried out in combination with an agglomeration or granulation of powdered products. Furthermore, such reactors may be used for other physical or chemical reaction processes, such as certain polymerization processes.
In principle, the mechanism of fluidization is that the particles contained in the powdered product overlying the perforated supporting plate are brought in a kind of floating, air-borne condition by means of the upwardly directed fluidization gas flow, so that the material behaves in a manner like a boiling liquid.
Fluidized bed reactors are constructed for discontinuous as well as continuous operation. In the simple case of discontinuous operation, in which a quantity of powdered material is processed for a certain period and is, subsequently, removed in one operation by evacuation of the apparatus, a single-stage reactor may often be used comprising a single powder supporting plate in the chamber.
In continuous operation, two main types of fluidized bed reactors are used, viz. the so-called back-mixed fluid bed reactor, or the plug-flow fluid bed reactor.
In the back-mixed type of fluid bed reactor, the composition of the powdered product is the same throughout the fluidized bed or layer and is identical to the composition of the final product. This reactor type will be useful for processing a starting material which is not in itself suitable for fluidization, provided that a sufficient distribution of the material across the fluidized bed can be obtained by feeding the material.
In the plug-flow reactor type, which may be constructed so that the fluidized bed has a very great length relative to its width, the composition of the powdered material varies from the inlet position towards the powder outlet of the apparatus. This reactor type is particularly useful when the process in the apparatus is taking place at a decreasing velocity, such as when drying to a low residial humidity. Fluidized bed reactors of the plug-flow type may also comprise a chamber having a circular cross-section, in which a spiral guiding wall is arranged upon the powder supporting plate, so that during the process, the powdered material flows continuously through the helical path formed by said wall after being fed in the central part of the chamber or at the outer wall thereof.
A process of a character similar to that taking place in reactors of the plug-flow type may also be obtained by a series arrangement of a number of single-stage reactors.
In some cases, fluidization may be combined with atomization of a liquid which may either, in so-called spray granulation, contain the product per se to be dried, or may serve for humidification or agglomeration of the fluidized powder.
This atomization is carried out by means of one or more atomizers which are normally arranged in the upper part of the chamber, and to which the liquid is supplied by inlet means extending through the ceiling of the chamber.
In case of products showing a very great variation with respect to particle sizes or shapes, or being otherwise difficult to fluidize, the fluidization may also be carried out in a vibrating, normally oblong chamber with product inlet means in one end and outlet means in the opposite end.
Frequently, it may be desired to use a fluidized bed reacting apparatus for processing products which show a tendency to adhesion and, consequently, to agglomeration even in a nearly dry condition, whereby depositions of such materials are often formed on the wall and ceiling faces of the reactor chamber, or on the perforated supporting plate. Such depositions are undesired and have to a certain degree led to restrictions with respect to the kind of products which may efficiently be processed in fluidized bed reactors.
In particular, depositions of the kind mentioned, on the upper side of the perforated plate may have the consequence that relatively high deposits are built up on the plate between the perforations to be passed by the fluidization gas, since the particle concentration in the fluidized bed is very high.